1. Field of the Invention
The present invention relates generally to a radio over fiber (ROF) link apparatus, and in particular, to an ROF link apparatus for transmitting a time division duplexing (TDD) wireless communication service without modulating a radio frequency (RF) band.
2. Description of the Related Art
Accompanying a variety, and a rapid increase, of information communication services, optical communication technology and wireless communication technology are being combined, thus increasing the necessity of a high-speed multimedia communication service.
Thus, research interests are concentrating on optical-wireless communication technology in which an ultra-high radio frequency is interlocked with a high-speed optical communication network to provide various kinds of bulk multimedia information communication services. By combining wired communication technology and wireless communication technology into an integrated technology of optical communication technology and wireless communication technology, a radio over fiber (ROF) technology is being vigorously studied.
Since an ROF system has many advantages, such as broadband channel capacity, low price, low power, and easy installation, operation, and management, the ROF technology provides appropriate solutions for high-speed wireless multimedia services for in-door applications such as airport terminals, shopping centers, and large-sized offices and out-door applications such as tunnels, narrow streets, and highways.
FIG. 1 is a block diagram of a conventional ROF link apparatus for a TDD wireless local area network (WLAN) service.
Referring to FIG. 1, the conventional ROF link apparatus includes a central station 100 and a base station 200. The central station 100 receives data from an upper layer, converts the received data to an RF signal for wireless communication, electro-optically converts the RF signal to an optical signal and transmits the optical signal to the base station 200 through an optical fiber. The central station 100 further receives upstream data generated in an RF manner from the base station 200 through the optical fiber, opto-electrically converts the received upstream data to an RF signal, converts the RF signal to baseband data, and transmits the baseband data to the upper layer. The base station 200 similarly, receives downstream data from the central station 100 through the optical fiber, opto-electrically converts the downstream data to an RF signal, and transmits the RF signal to a WLAN service terminal 300 through an antenna, and further receives upstream data from the WLAN service terminal 300, opto-electrically converts the upstream data to an optical signal, and transmits the optical signal to the central station 100 through the optical fiber. The central station 100 operates in TDD wireless communication protocol.
In the TDD wireless communication, the same frequency band is time divided and used for transmitting upstream data and downstream data. That is, an assigned frequency band is used to transmit downstream data in a specific time and used to transmit upstream data after the downstream data is transmitted. Thus, as the same frequency band is generally used to transmit upstream and downstream data, a TDD wireless system has better frequency usage efficiency than a conventional frequency division duplexing (FDD) wireless system. However, since a technique of processing data by dividing a short time period is required, the TDD wireless system has a relatively complex system configuration. Recently, TDD wireless systems are used in wireless services such as WLAN and mobile Internet.
When data is processed in the TDD wireless communication, as illustrated in FIG. 1, TDD data of a baseband is modulated to TDD data of an RF band using WLAN access points (APs) 101 and 102 operating in a TDD method included in the central station 100.
The central station 100 also includes an RF coupler/divider 103 for performing coupling and dividing operations to process an RF input and an RF output to and from the WLAN APs 101 and 102, and an opto-electrical converter 105 and an opto-electrical converter 104 for transmitting data through the optical fiber.
Each of the WLAN APs 101 and 102 includes an Ethernet switching unit for connecting with the upper layer, a baseband processing unit for converting baseband data input through the Ethernet switching unit to RF data, and an RF transceiver module for transmitting the converted RF data to the RF coupler/divider 103. Although only a downstream operation of the WLAN APs 101 and 102 has been described, and as the upstream operation is opposite to the downstream operation, the upstream operation need be not discussed in detail herein. However, it would be well within the knowledge of those skilled in the art to understand the upstream operation based on the discussion of the downstream operation discussed herein.
The base station 200 includes an opto-electrical converter 106 for converting an optical signal received from the central station 100 to an electrical signal, an electro-optical converter 107 for converting an electrical signal to an optical signal and transmitting the converted optical signal to the central station 100, and an RF amplifier 108 for amplifying downstream data (RF signal) converted to the electrical signal using the opto-electrical converter 106 to output through the antenna, and amplifying a weak RF signal received through the antenna to transmit to the central station 100 through the electro-optical converter 107.
As described above, the ROF link apparatus has a structure in which the central station 100 and the base station 200 are connected through the optical fiber, i.e., an optical relay structure of a general wireless communication system. However, since the TDD method is applied to the ROF link apparatus, the WLAN APs 101 and 102 are disposed in the central station 100.
Operations of the WLAN APs 101 and 102 will now be described. During a specific transmission time, an RF signal output from the WLAN AP 101 is modulated to an optical signal by the electro-optical converter 104 and transmitted to the base station 200 through the optical fiber. The transmitted optical signal is converted to an RF signal by the opto-electrical converter 106, amplified by the RF amplifier 108, and propagated through the antenna.
The operation described above is performed during the transmission time by the ROF link apparatus using the TDD method.
During a reception time, a weak upstream RF signal input through the antenna is low-noise amplified by a low noise amplifier (LNA: not shown) of the base station 200, amplified to an RF signal having a constant level by the RF amplifier 108, converted to an optical signal by the electro-optical converter 107, and transmitted to the central station 100 through the optical fiber. The transmitted optical signal is converted to an RF signal by the opto-electrical converter 105, input to the WLAN AP 102 through the RF coupler/divider 103, and processed by the WLAN AP 102.
As described above, when the WLAN APs 101 and 102 are disposed in the central station 100 for the TDD wireless communication of the ROF link apparatus, and a delay according to the length of the optical fiber may occur in the time division processing of the TDD method, throughput of service data may be reduced according to the length of the optical fiber, or the TDD system may not operate at all because of the amount of an optical signal loss.
When a signal is transmitted through a single mode optical fiber, a propagation delay time is around 5 μs/km is introduced in a typical environment. In a TDD WLAN system, a subsequent data frame can be transmitted only if an acknowledgement message, indicating that the other party has received a data frame without an error, is received within tens μs after an AP transmits the data frame. Thus, if an optical signal propagation delay time is too longer because the length of an optical fiber is longer than a predetermined distance, the possibility the acknowledgement message is received within a pre-defined time is high; thereby disabling a normal operation.
In the TDD WLAN system, based on the structure of the ROF link apparatus, a serviceable range is limited due to the optical signal propagation delay time and not an optical fiber link propagation loss. Thus, extension of the serviceable range is not sufficiently utilized, which is an advantage of ROF systems.